The present invention relates to polymeric compositions that can be used to retain product quality and improve shelf life of oxygen sensitive materials, and to intermediate shaped structures, e.g., films, coatings, 3-dimensional solids, fibers, webs and the like which contain said composition, as well as to shaped products into or onto which said composition or structure are incorporated or applied to, be part of or attached to the container structure.
The composition is composed of a polymeric composition which contains porous particulate having oxygen scavenging metal contained therein. Specifically, the composition utilizes a high surface area microporous particulates which have been coated with metal atoms selected from magnesium, calcium, tin or one of the transition metals from scandium to zinc or mixtures thereof. The metal coated particulate containing oxygen scavenging composition of the present invention provides effective absorption of oxygen from the interior of a container without adversely affecting the color, taste or smell of the packaged material contained therein. Further, the resulting composition is thermally stable and does not emit volatile by-products to contaminate the packaged material.
The subject oxygen scavenging composition has the ability to effectively chemically combine with oxygen in contact therewith, such as from the interior of a container, without undue migration of the oxygen scavenging metal-coated material out of the matrix. The migration stability of the metal and its metal-coated material is of particular advantage in that it significantly reduces or eliminates adverse effects on the color, taste, or smell of articles in contact with the matrix composition.
In order to enhance preservation, it is standard practice to package food and other materials within laminated packaging material that generally includes a barrier layer, that is, a layer having a low permeability to oxygen. The sheet material can be thin, in which event it is wrapped around the material being packaged, or it can be sufficiently thick that it forms a shaped container body that is provided with a lid or other separate closure. The polymeric sheet material may constitute some or all of the interior exposed surface area of the container.
It is known to include an oxygen scavenger in sheet material. The oxygen scavenger reacts with oxygen that is trapped in the package or that permeates into the package. This is described in, for instance, U.S. Pat. Nos. 4,536,409 and 4,702,966 and the prior art discussed in these references. U.S. Pat. No. 4,536,409, for example, describes cylindrical containers formed from such sheet material and provided with metal lids.
When the container is formed of a glass or metal body and is provided with a hermetically sealed metal closure, the permeation of oxygen through the body and the closure is theoretically impossible because of the impermeability of the materials forming the body and closure. As a practical matter, metal cans can reliably prevent oxygen ingress. However, some oxygen ingress may occur by diffusion through the gasket or the like positioned between a container body and its lid. It has long been recognized that when conventional containers of these types are used for the storage of oxygen sensitive materials, the shelf life of the stored materials is very limited. The quality of the packaged material tends to deteriorate over time, in part because dissolved oxygen typically is present in the pack from the time it is filled; and in part due to oxygen ingress which occurs during storage.
When the container is in the form of a can, the can end or other closure in many instances includes push components or pull components which are intended to be, respectively, pushed or pulled in order to allow removal of the fluid or other material in the container without removing the entire closure from the container. These push or pull components are often defined by discontinuities or lines of weakness in the panel of the closure. Problems that can arise at these lines of weakness or discontinuities include the risk of permeation of oxygen into the container and the risk of corrosion of the metal where the normal protective lacquer coating is ruptured at the lines of weakness or at the discontinuities.
It would be very desirable to be able to significantly improve the shelf life while continuing to use conventional materials for the formation of the container body, the container closure and, where applicable, the gasket between the body and closure.
Various types of oxygen scavengers have been proposed for this purpose. For example, it is well known to package iron powder in a sachet for use with dry foods. See Mitsubishi Gas Chemical Company, Inc.'s literature titled "Ageless.RTM.--A New Age in Food Preservation" (date unknown). However, these materials require the addition of water soluble salts to enhance the oxygen scavenging rate and, in the presence of moisture, the salts and iron tend to migrate into liquids, producing off-flavors. Similarly, U.S. Pat. No. 4,536,409 issued to Farrell et al. recommends potassium sulphite as a scavenger, with similar results. U.S. Pat. No. 5,211,875 issued to Speer et al. discloses the use of unsaturated hydrocarbons for use as oxygen scavengers in packaging films.
It is known in the art that ascorbate compounds (ascorbic acid, its salts, optical isomers, and derivatives thereof) can be oxidized by molecular oxygen, and can thus serve as components of an oxygen scavenging formulation, for example, as a component of a closure compound. For example, U.S. Pat. No. 5,075,362, issued to Hofeldt et al., discloses the use of ascorbates in container closures as oxygen scavengers.
U.S. Pat. No. 5,284,871 issued to Graf relates to the use of an oxygen scavenging composition made of a solution of a reducing agent and dissolved species of copper which are blended into foods, cosmetics and pharmaceuticals. Cu.sup.2+ ascorbate is used in the examples. The reference indicates that relatively high level of Cu.sup.2+ (.about.5 ppm) are required in the food for scavenging to be effective but indicates that small amounts of the Cu.sup.2+ can combine with oxygen in food to cause food spoilage. In order to avoid spoilage, one is required to reduce the amount of headspace O.sub.2 or partially flush the container with an inert gas (Col. 5, lines 32-39). A paper by E. Graf, "Copper (II) Ascorbate: A Novel Food Preservation System", Journal of Agricultural Food Chemistry, Vol. 42, pages 1616-1619 (1994) identifies copper gluconate as a preferred raw material.
It is also well known in the scientific literature (See "Polymer Compositions Containing Oxygen Scavenging Compounds", Teumac, F. N.; et al. WO 91/17044, published Nov. 4, 1991, filed on May 1, 1991) that the oxidation rate of ascorbate compounds can be increased significantly by the use of catalysts. Typical oxidation catalysts for ascorbic acid and its derivatives are water soluble transition metal salts. When such catalysts are combined with an ascorbate compound in a polymeric matrix, e.g., a PVC closure formulation, they are effective in catalyzing the oxidation of the ascorbate compound, and increase the oxygen scavenging rate of the ascorbate.
In each of the above references, the active agents of the oxygen scavenging systems utilize organic materials which would produce by-products (e.g., aldehydes, acids, ketones) of the oxidation process. These by-products are known to adversely effect a wide range of packaged material.
Copper zeolite powders have been used in tubular reactors under relatively high temperature conditions, such as 140.degree. C. or higher, to remove small quantities of oxygen contained in gas streams. See "Activation of Copper Dispersed on a Zeolite for Oxygen Absorption" by Sharma and Secham, Chem. Modif. Surf., 3 (Chem Modif Oxide Surf), 65-80. Agents which perform at such elevated temperatures have not been deemed appropriate for use in food or food container applications, as foods are typically maintained at relatively low temperatures and are not exposed to temperatures above 120.degree. C. for any extended time.
Further, copending U.S. application having U.S. Ser. No. 08/764,874, filed Dec. 3, 1996, teaches that certain zeolites having ion-exchange capacity can be used as an exchange medium whereby copper and other oxygen scavenging metals are adhered to the zeolite by ion-exchange technique. The capacity to scavenge oxygen of the resultant product is substantially limited by the exchange capacity of the material. Thus, because the exchange capacity is normally low, the amount of metal contained in the material is lower than desired to provide a high capacity oxygen scavenging material which can provide extended shelf-life to a product.
It is highly desired to provide an effective oxygen scavenging system suitable for packaging applications which has good oxygen absorption capabilities and capacity and which does not itself or by its by-products, provide material which adversely effects the color, taste or smell of the packaged material.
It is further desired to provide an effective oxygen scavenging system which has the active scavenger agent contained within a carrier and the agent still provides effective scavenging capacity.
It is further desired to provide an effective oxygen scavenging system which is thermally stable and, thereby, capable of allowing the packaged system to undergo pasteurization or sterilization.